Those still using Atari STs have an advantage in that the machine comes with a built-in MIDI interface, but in the case of the other computers popularly used in music, you have to provide your own. PC interfaces generally come as plug-in cards, whereas Mac interfaces are invariably boxes that connect either to the modem or printer port. In the case of the PC, it's as well to check that the interface will run alongside any other cards you have installed, such as soundcards (and music software for that matter), because there can be problems with conflicts or incompatible software drivers. If you buy your interface from a store that has good technical support, then you should be able to confirm this at the time of purchase, and even if you can get a cheaper deal from a box-shifting outlet, I'd still recommend you buy from a specialist unless you really know what you are doing. One of the most successful PC MIDI interfaces is the Roland MPU401, and you'll find that many of the third party cards state that they have MPU401 compatibility.
A possible alternative to the conventional MIDI interface is to buy a synth module with an interface built in, and there are already several GM-style modules on the market that can link directly to a PC or Mac. However, I didn't start this article just to tell you about basic MIDI interfaces, but rather to introduce the concept of the multi-port MIDI interface for those needing more than 16 channels.
When MIDI was first introduced, 16 channels seemed perfectly adequate, but we now have multitimbral sound modules that can gobble up all 16 channels in one bite (pun intended!). This poses a problem if you want to use, and access several different synth modules at the same time. The solution is to gain more channels, but how is this possible when MIDI is limited to 16?
The answer lies in the multi-port MIDI interface, where there are two or more MIDI output sockets, each of which has its own set of MIDI channels. (This shouldn't be confused with an interface that has multiple Thru outputs, as these simply send the same information via all outputs to save you having to buy a separate Thru box.) In order to make use of a multi-port interface, you have to use sequencing software that supports multi-port operation, but all the current market leaders (Steinberg, Emagic, MOTU, Opcode, and so on) have this facility. Within the software is a setup section that allows you to tell the system what type of interface you are using, and the most commonly supported models for the Mac (in addition to anything the software company might supply) are the MOTU MIDI Time Piece, the MOTU MIDI Express, and the Opcode Studio series interfaces. Software-specific interfaces for use with Ataris include Emagic's Log 3, C-Lab's Unitor and Xport, and Steinberg's Midex, although there's now a Log 3 for the Mac as well. The more sophisticated of these interfaces include SMPTE read/write facilities, SMPTE to MTC conversion, and a degree of programmable MIDI routing, as well as multiple MIDI Ins which can be merged. If you're running a system which includes a software editor/librarian package, then the ability to route the MIDI Outs of your modules back to the computer without repatching is very valuable.
This brings us back to the software -- how are these extra channels named and used? Normally, the separate MIDI output ports are numbered A, B, C, D, and so on, and inside the sequencer software, the MIDI channel number may be followed by a letter. For example, MIDI channel 8 on MIDI port B would simply be Ch 8B. I've created a multiple column list in my own studio which shows me which synths are connected to which MIDI ports, and on which channels they are set to operate. Obviously a 16-part multitimbral instrument will take up a whole port if you intend to use all 16 channels at once, but 8-part modules or non-multitimbral modules can share a port in the conventional manner.
It sounds straightforward enough so far, but there are one or two things it pays to be aware of, the most important of which is that with all the interfaces I've used, MIDI clock only comes out of port A. In other words, if you need to sync a drum machine or a second sequencer using MIDI clock, you have to connect it to port A.
Mac users should also note that you can have one MIDI interface connected to the modem port and another connected to the printer port -- these can both be used at once. This makes defining the MIDI channel a bit more complicated, because now we have the channel number, the port letter, and an M for modem, or a P for printer. The actual order of letters and numbers may vary from sequencer to sequencer, but the idea is simple enough. Due to its two outputs, the Mac can support up to 32 channels using two single-port interfaces, but at the other extreme, you could use two 8-output interfaces to give you 16 sets of 16 channels, which should cover most eventualities.
One further benefit of using a multi-port interface is that MIDI data can be spread over the different ports to avoid data bottleneck problems. Each port is, in theory, as capable of providing MIDI data as a single conventional MIDI interface, but if you were to use all 16 ports, each running 16 busy channels, you might be asking rather a lot of your computer!